1. Field of the Invention
The present invention relates to an optically active phosphine oxide carboxylic acid and the method of producing the same. The phosphine oxide having its asymmetric center at a phosphorus atom is a compound especially useful as a precursor of an optically active phosphine compound which may be used as a ligand of an asymmetric synthesis catalyst.
2. Description of the Prior Art
The oldest known method of producing an optically active phosphine oxide is by reacting an .alpha.-bromocamphor sulfonic acid to a methyl ethyl phenyl phosphine oxide, the produced salt being separated and crystallized, and then treated by ammonia to obtain an optically active phosphine oxide. (J. Meisenheimer, L. Lichtenstadt, Ber., 44, 356 (1911)).
The above-mentioned method utilizes the difference in the solubility of a diastereomer generated by the acidic of the camphor sulfonic acid and the basic of the phosphine oxide. However, since a camphor sulfonic acid (camphor) which is a natural product is used as the separating agent, there is a drawback that camphor with sufficient purity may not be obtained when isolating the same from natural products.
Therefore, another known method of obtaining an optically active phosphine oxide comprises the following steps. A thionyl chloride and an optically active 1-menthol is reacted under the existence of a triethylamine to a phosphinic acid of a different alkyl group such as a methylphenyl phosphinic acid, so as to obtain a diastereomer salt of a menthyl methyl phenyl phosphinate. The salt is separated and crystallized, and treated by base to obtain an optically active menthyl methyl phenyl phosphinate. Moreover, an optically active phosphine oxide is also known to be obtained by reacting a Grignard reagent. (Homogeneous Catalysis II, Adv. Chem. Ser., No. 132,274 (1974)).
This method utilizes an equivalent stoichiometric amount of 1-menthol which is an asymmetric source, and introduces the same by a covalent bond to a phosphinic acid which is a reaction substrate. However, this method uses a large amount of expensive asymmetric source, and moreover, the portion of the asymmetric source introduced by the covalent bond after the reaction must be removed and collected by a chemical process of some kind. Therefore, such asymmetric reaction may be used for a compound having high additional values such as a prostaglandin, but is not practical as a general industrial process.